Historically and conventionally artist's canvas or pictures painted on canvas have been stretched over wooden frames by firmly attaching the edges of the canvas sheet to the frame, upon which it is being stretched, by means of tacks, staples, nails, or clamps spaced at short distances from each other. Slackness, bulges, wrinkles, and deformations in the canvas that are normally caused by changes due to gravity, temperature, humidity or deformations of the frame, to which the canvas sheet is attached, could be corrected by repeating the tensioning operation which meant separating the canvas sheet from the frame and reattaching it in a laborious and time consuming operation requiring considerable skill and practice. Customary methods of achieving minor corrections in tensioning of canvas stretched over a frame have involved changes in construction of the corner pieces to allow varying their size or wedging apart the corner joint of the frame, generally called "keying out". Such localized adjustments necessarily lead to higher stresses in the corner regions of the canvas with possible distortions and damage to the painting.
Illustrative of the prior art for tensioning canvas over frames are: U.S. Pat. No. 142,232 by Holly, where an outer frame is held by dowels and springs against an inner frame, the effect being "keying out" of the canvas sheet with most of the tensioning forces at the spreadable corners and no adjustment of the spring forces in situ provided; U.S. Pat. No. 2,456,225 by Thomas, where helical springs exert pull to hold tack-like projections of a ferrule along the frame to attach the canvas, the tensioning of the canvas to be done by conventional "keying out"; U.S. Pat. No. 3,133,375 by Myren, with a construction of the mitered corner joints of the frame that permits "keying out" at the corners by inserting a wedge to a needed depth, resulting in tensioning at the corners of the canvas sheet; U.S. Pat. No. 3,529,653 by Fey, where the frame sides have L-shaped cross sections, the canvas being clamped rigidly around their flanges by means of channels fitting over them, the corner structures with toggle braces for stretching the assembly at the corners; U.S. Pat. No. 3,914,887 by Newman, where the edges of the canvas are held tightly in the channel-shaped inside of the frame sides, screw mechanism enabling the corner members to be adjusted in their distance from the sides of the frame and screw mechanisms to govern the tension at the corners, thus after mounting only allowing manipulation at the corners. Suffice it, the prior art does not permit spontaneous adjustment of the tension in the canvas along the circumference of the frame, nor spontaneous maintenance of tension over the sheet, nor setting of the tension by springs to a value desired, that is automatically maintained in the tension of the mounted canvas, nor does it permit later adjustment of springs in situ to another desired tension value.